In the current system and functionality description for the Enhanced uplink, in the 3GPP specification, the involved mechanism for rate selection is described from a user equipment (UE) perspective. Basically the UE should use the minimum rate selected among “system granted rate” or “UE evaluated maximum rate based on power consumptions”.
When the UE is limited by its own maximum Tx power, it has to evaluate which rate it can support (the UE does this evaluation all the time in order to find out whether it is power or grant limited) in order not to end up in a power shortage situation. With a power shortage situation is meant that the UE does not have enough power to transmit a selected block, i.e. the UE has not failed to transmit a specific enhanced dedicated channel transport format combination (E-TFC), but the transport block is transmitted with such low power that the Node B cannot detect the transmission correctly.
When the UE starts to experience power shortage, the long term (and preferred) solution is to change the used E-TFC so that the available power is enough for a proper transmission. This action takes some transmission time intervals (TTIs) to accomplish. During this execution time, the UE starts to temporarily reduce the power allocation to the Enhanced dedicated physical data channel (E-DPDCH).
The E-TFC evaluation is based on the used power for the dedicated physical control channel (DPCCH) together with power offset values for each existing transport format (E-TFC), i.e. βed. Based on the DPCCH power and βed's, the UE evaluates which E-TFC it can support. The intermediate temporal power reduction is carried out by reducing the βed factor on slot basis.
Another function involved is the outer-loop power control (OLPC). In enhanced uplink (EUL), the input to the OLPC is the information on the number of Transmission Attempts (TA). An OLPC up-step is executed when the targeted number of transmission attempts is exceeded. A radio link control (RLC) re-transmission is executed if the maximum number of TAs is exceeded. Information on the number of TAs is available to the radio network controller (RNC) for correctly received blocks or in the case where the hybrid automatic repeat request (HARQ) completely fails to decode the blocks; i.e. the blocks are still un-decoded after the maximum number of TAs. In TTIs where the blocks are received incorrectly by the HARQ, no message is sent to the OLPC. The OLPC will not have any information to work on during that TTI, and consequently not take any action.
When the UE has a higher grant than it can use, the UE has to trust its own evaluation of which format that can be used without ending up in power shortage. It is important that the UE succeeds with this evaluation, since, if it fails and runs into power shortage, extra transmissions will most likely be needed (i.e. more transmissions than the target number of transmissions). When the targeted number of transmissions is not fulfilled, the OLPC will increase the signal-to-interference ratio (SIR) target.
More specifically, a UE in power shortage will likely not get through with the first TA (assuming a transmission target of one transmission in this example), and the SIR-target will be increased by the OLPC. At next TTI border, a new transport block (TB) size is selected by the E-TFC selection functionality and a new required-power-per-block estimate is performed. Still short of power, the loop ‘lack of power’→‘OLPC up-step’→‘increased SIR-target’→‘new E-TFC selected’ might continue until a minimum E-TFC is selected. The problem is severe when the UE is transmitting with the smallest available E-TFC, thus not able to select any smaller format that potentially could improve the situation. During this time, the SIR-target might have been increased several dBs. Unnecessarily high SIR levels have impact on system throughput, but more alarming, severe impact on end-user throughput too.
In extreme cases when the UE only transmits data infrequently, e.g. only has power enough to transmit data during short times, the SIR-target can increase rapidly, which is undesired. As a consequence, the UE gets it even more difficult to transmit any data again since the power allocation to the control channel has been increased.
In power shortage where the SIR-target has been raised one (or several times), i.e. having a marginal with respect to the foreseen SIR-target value, this rise/marginal might show up not to be sufficient when the UE tries to transmit another E-TFC (with the outcome of increased SIR-target once again). In other words, the current E-TFC selection mechanism in combination with an OLPC is not optimal.
Currently, the UE can signal the power situation to the Node B (upon request of the Node B) by means of a power headroom report, i.e. ‘UE Power Headroom’ (UPH) in the uplink (UL) Scheduling information. The UPH field indicates the ratio of the maximum UE transmission power and the corresponding DPCCH code power. This signalling provides information on DPCCH power levels, and by knowledge of the involved beta-factor, you got information on all powers. The shortcomings are that neither any power-dependent trigger mechanism is defined, nor that periodical signalling is realistic due to the power consumption, nor that the UE can decide when to inform the Node B about its power situation. Additionally, typical time resolution of the currently used methods (system polls or periodical signalling) is in the order of several TTIs, which is too long to be optimal.